| Titanium alloy has the characteristics of high specific strength,good performance in medium and low temperature,corrosion resistance,etc.,and is widely used in the fields of aerospace,automobile industry,sports equipment and petrochemical industry.However,it has disadvantages such as poor wear resistance,low hardness,and poor high-temperature performance,which makes titanium alloy parts prone to surface scratches and corrosion in harsh working environments,especially when used as processing materials for thin-walled parts,they are prone to fractures and surface cracks.In recent years,in order to strengthen and modify the surface of titanium alloys,researchers at home and abroad have tried to use various surface technologies to treat titanium alloys.Among them,laser alloying technology has the characteristics of high energy density,small heat-affected zone,controllable thickness,and dense alloying layer,especially the metallurgical bonding of alloying layer and substrate,and the ability to selectively treat specific surfaces of workpieces.It is considered to be one of the most promising technologies at present.Through the adjustment of laser process parameters and the design of alloying material system,an alloyed layer with excellent performance can be prepared on the surface of titanium alloy.In this paper,nickel-coated graphite(G@Ni)is used for laser alloying of TC4 alloy matrix.During the alloying process,complex chemical reactions occur in the molten pool.Ti reacts with C to form a ceramic strengthening phase,and Ni,Ti,and Al can form a variety of intermetallic compounds.Because the in-situ formed strengthening phase has better compatibility with the matrix phase of the alloyed layer,the hardness,wear resistance and high temperature resistance of the titanium alloy matrix have been significantly improved.This paper discusses the effects of laser power,scanning speed and other process parameters on the macro morphology,microstructure,microhardness and wear resistance of the alloyed layer,and compares the effects of nano-rare earth oxides Nd2O3 and La2O3 on the alloying The influence of phase composition,microstructure and wear resistance of the layer,and the high-temperature oxidation,salt-coated hot corrosion and thermal fatigue behavior of the alloyed layer at 800? were studied,and the formation mechanism and strengthening mechanism of the phases in the alloyed layer was analyzed.The G@Ni alloying layer is mainly composed of TiC,?-Ni,Ni3Al,NiTi,NiTi2 and other phases.To ensure proper laser energy density,an alloyed layer with good surface quality and no defects such as pores and cracks can be prepared.Laser alloying is carried out under the conditions of a laser power of 1.4 kW and a scanning speed of 12-18 mm/s.As the scanning speed increases,the structure size in the alloyed layer becomes smaller.When the scanning speed is 18 mm/s,the average microhardness of the alloyed layer is the highest,which is 1357.7 HV0.2.When the scanning speed is 15 mm/s,the wear of the alloyed layer is the smallest,and the wear resistance under the same conditions is increased to 13.79 times that of the base material.When the scanning speed is 15 mm/s and the laser power is increased from 1.2 kW to 1.6 kW,the size and density of the microstructure in the alloyed layer increase.When the laser power is 1.4 kW,the average hardness of the alloyed layer is the highest,1365.7 HV0.2,and the wear resistance is best increased to 13.79 times that of the titanium alloy substrate.Nano-rare earth oxides Nd2O3 and La2O3 have little effect on the phase composition of the alloyed layer,but they can refine the structure of the alloyed layer and improve the microhardness and wear resistance,but the amount of addition should not be too large.Otherwise it will increase the dilution rate of the alloyed layer,coarsen the structure,reduce the microhardness,and limit the further improvement of wear resistance.In this paper,when 1.5 wt.%n-Nd2O3 is added,the alloyed layer wear loss is only 0.0018 g,which is 14.29%lower than the alloyed layer without rare earth oxides(0.0021 g)under the same conditions,and the wear resistance is improved to the same 14.56 times of the base material under the conditions.When the addition amount of n-La2O3 is 1.5 wt.%,the alloyed layer wear loss is only 0.0017 g,which is 22.73%lower than the alloyed layer without rare earth oxides(0.0022 g)under the same conditions,and its wear resistance is increased to 15.41 times that of TC4 matrix.The high temperature performance of the alloyed layer is studied.After cyclic oxidation at 800? for 100 hours.the oxide film on the surface of the alloyed layer did not appear obvious peeling during the entire oxidation cycle.The oxidation weight gain was 14.571 mg/cm2.which was only 1/2 of the substrate(30.458 mg/cm2).the oxidation weight gain of the alloyed layer with n-Nd2O3 added is 6.244 mg/cm2.which is 1/5 of the matrix,and the alloyed layer with n-La2O3 added has an oxidation weight gain of 3.317 mg/cm2,which is 1/10 of the matrix,showing good resistance to high temperature oxidation.The TC4 matrix was severely corroded in a short period of time under the salt-coated hot corrosion condition at 800?,and the corrosion products formed were thick and peeled seriously.In contrast,the G@Ni alloyed layer improves the thermal corrosion resistance of the substrate,and the alloyed layer added with n-Nd2O3 or n-La2O3 has better thermal corrosion resistance,indicating that rare earth can improve the thermal corrosion resistance of the coating The role of performance.The thermal cycle life of the alloyed layer(21 times)is longer than that of the TC4 matrix(8 times)under the heat and cold cycle of 800?,and the alloyed layer with n-Nd2O3 or n-La2O3 added starts after 27 and 28 cycles The initiation of cracks indicates that the thermal fatigue resistance of the alloyed layer is significantly higher than that of the matrix,and an appropriate amount of rare earth oxide helps to further improve its thermal fatigue performance. |
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